Abstract
The changes in the concentration and composition of biomass-burning organic aerosol (OA) downwind of a major wildfire are simulated using the one-dimensional Lagrangian chemical transport model PMCAMx-Trj. A base case scenario is developed based on realistic fire-plume conditions and a series of sensitivity tests are performed to quantify the effects of different conditions and processes. Temperature, oxidant concentration and dilution rate all affect the evolution of biomass burning OA after its emission. The most important process though is the multi-stage oxidation of both the originally emitted organic vapors (volatile and intermediate volatility organic compounds) and those resulting from the evaporation of the OA as it is getting diluted. The emission rates of the intermediate volatility organic compounds (IVOCs) and their chemical fate have a large impact on the formed secondary OA within the plume. The assumption that these IVOCs undergo only functionalization leads to an overestimation of the produced SOA suggesting that fragmentation is also occurring. Assuming a fragmentation probability of 0.2 resulted in predictions that are more consistent with available observations. Dilution leads to OA evaporation and therefore reduction of the OA levels downwind of the fire. However, the evaporated material can return to the particulate phase later on after it gets oxidized and recondenses. The sensitivity of the OA levels and total mass balance on the dilution rate depends on the modeling assumptions. The high variability of OA mass enhancement observed in past field studies downwind of fires may be partially due to the variability of the dilution rates of the plumes.
Highlights
Particles and trace gases emitted or produced in the atmosphere have serious impacts on human health
Changes in Biomass burning organic aerosol (bbOA) concentrations depend on the relative balance between evaporation of fresh biomass burning primary organic aerosol driven by dilution [11] or by fragmentation reactions creating higher-volatility species, and the formation of biomass burning secondary organic aerosol from the oxidation of semi-volatile (SVOCs), intermediate volatility (IVOCs) and volatile organic compounds (VOCs)
AAtt lloonnggeennoouugghhttiimmeessccaalleesstthheepplluummee iissddiilluutteeddeennoouugghhaannddtthhee∆ΔOΟAΑ//Δ∆CCOO aapppprrooaacchheess tthhaatt ooff tthhee bbaacckkggrroouunndd. These results suggest that the apparent increase in ∆OA/∆carbon monoxide (CO) is quite sensitive to the assumed horizontal dilution rate constant
Summary
Particles and trace gases emitted or produced in the atmosphere have serious impacts on human health. Changes in bbOA concentrations depend on the relative balance between evaporation of fresh biomass burning primary organic aerosol (bbPOA) driven by dilution [11] or by fragmentation reactions creating higher-volatility species, and the formation of biomass burning secondary organic aerosol (bbSOA) from the oxidation of semi-volatile (SVOCs), intermediate volatility (IVOCs) and volatile organic compounds (VOCs) The study of these changes is facilitated by normalizing the OA concentrations to a co-emitted species such as carbon monoxide (CO), assumed to be practically inert in the measurement time scales [12,13]. Bian et al [25] performed simulations of the evolution of ambient bbOA concentrations, showing that the fire area, the mass emissions flux, and the atmospheric stability strongly modulate initial plume concentrations and plume dilution rates They concluded that while the measured OA enhancement ratio may be close to unity, there is still significant SOA formation in these plumes which is balanced by POA evaporation. Atmosphere 2021, 12, 1638 pathways (functionalization/fragmentation split), temperature, and dilution rates (vertical and horizontal) are investigated
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